Temporary coatings or material layers have been used to hold and protect parts, especially delicate and easily deformable parts such as honeycomb cores, during machining, processing and shipment. Such a protective layer must have sufficient adhesion to the part so that the layer will not be readily peeled away or removed from the part during handling, but yet it may be removed easily and completely after machining processing or shipment of the part is complete.
In the aerospace field, structural honeycomb has many applications and often must be machined to close tolerances to form intricate contours. Such machining is difficult or impossible to accomplish unless the thin-walled honeycomb core is in some manner supported or rigidly stabilized.
To stabilize honeycomb for machining, use has previously been made of significant quantities of a compound containing mixtures of certain synthetic water-soluble waxes, mica, fiberglass and alkali metal carbonates which solidify at low temperatures and are suitable for removal by water or acid washing as disclosed in U.S. Pat. No. 3,176,387. This patent teaches that the entire volume of the voids of a honeycomb core are filled with the compound before machining. Such waxes are generally made of polyethylene glycol but suffer from the disadvantage that, even through the waxes are water soluble and may be removed by water, without a thorough cleaning of the entire structure, a wax residue remains on the core. This residue, if not removed, prevents proper adhesive bonding generally required in later manufacturing operations using the honeycomb itself. Also, the polyglycol is not suitable for use with non-metallic cores such as fiberglass or Nomex. It also should not be used on parts which are spliced with foam adhesive since it cannot be readily removed from the pores of the foam. Thorough cleaning of the surface with water to permit complete removal of all of the wax is wasteful of natural resources and is generally not economical.
In U.S. Pat. No. 3,064,345, a temporary air-tight layer is disclosed in which the layer is obtained by using a resin layer within cellular material so that the material may be more readily secured in place for shaping or machining. The patent teaches that, when molten polyglycol wax is poured into heated perchloroethylene, two layers are formed with the polyglycol layer being on top. The cellular structure is placed in the heated liquid bath and allowed to settle to the desired level. Upon cooling, the polyglycol wax solidifies to provide a rigid air-tight chamber in the cellular structure which enables the structure to be held for machining. The polyglycol wax is then removed by immersion in or spraying with water.
Since the method of U.S. Pat. No. 3,064,345 uses perchloroethylene, a chlorinated hydrocarbon solvent, the method must be performed carefully to avoid fire hazards. Moreover, the method in this patent would appear to be suitable for use only with relatively small cellular structures.
In U.S. Pat. No. 2,905,064, it has been proposed to stabilize honeycomb with ice. In addition, U.S. Pat. No. 3,413,708 disclosed the use of ice in combination with a plurality of pressure pads to permit the machining of more than one structural surface. Although the use of ice has been successful in stabilizing thin-walled honeycomb during machining, the time required to freeze the ice and to thereafter re-freeze it for additional machining operations, as well as the formation of distortion which may be caused by thermal expansion or contractions during the freezing and thaw cycles, make it difficult to use ice to maintain required tolerances for various honeycomb parts.
U.S. Pat. No. 3,046,175 discloses a method of chemical forming of a honeycomb core. The honeycomb structure is inserted into a rubber-like coating material and then cured. The open cells of the honeycomb are then completely filled with a hot melt composition, followed by chemical etching using an acid solution in which the exposed core material is removed. The protective material is then mechanically removed. This method is very time consuming and requires the use of large quantities of hazardous, strongly acidic solutions.
Additional techniques which have been used to hold structural material for machining purposes are disclosed in U.S. Pat. Nos. 2,855,664, 2,937,437, 3,003,002 and 3,093,370.
In the handling of thin walled structures such as honeycomb cores, it is often necessary or desirable to avoid the application of heat to such structures because of adverse physical effects thereon. At elevated temperatures, the adhesive bonding between the cell structures of a honeycomb core may soften and cause significant deformation of the structure. Thus, in the preparation of honeycomb for machining using a thermoplastic resin, elevated curing temperatures of 350.degree. F. coupled with extended curing times for the resin must be avoided. Moreover, it does not adhere well to a honeycomb structure. Other thermoplastic resins and waxes have been used but they generally suffer from the disadvantage that they may be corrosive to the metal structure or the residue can be removed only by the use of extensive cleaning with the use of water, acid solutions or organic solvents.